Date:   Fri, 23 Apr 2021 08:01:54 +0000
From:   HORIGUCHI NAOYA(堀口　直也) 
        <naoya.horiguchi@....com>
To:     Mike Kravetz <mike.kravetz@...cle.com>
CC:     Michal Hocko <mhocko@...e.com>,
        Muchun Song <songmuchun@...edance.com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "osalvador@...e.de" <osalvador@...e.de>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>
Subject: Re: [PATCH] mm: hugetlb: fix a race between
 memory-failure/soft_offline and gather_surplus_pages

On Thu, Apr 22, 2021 at 08:27:46AM +0000, HORIGUCHI NAOYA(堀口 直也) wrote:
> On Wed, Apr 21, 2021 at 11:03:24AM -0700, Mike Kravetz wrote:
> > On 4/21/21 1:33 AM, HORIGUCHI NAOYA(堀口 直也) wrote:
> > > On Wed, Apr 21, 2021 at 10:03:34AM +0200, Michal Hocko wrote:
> > >> [Cc Naoya]
> > >>
> > >> On Wed 21-04-21 14:02:59, Muchun Song wrote:
> > >>> The possible bad scenario:
> > >>>
> > >>> CPU0:                           CPU1:
> > >>>
> > >>>                                 gather_surplus_pages()
> > >>>                                   page = alloc_surplus_huge_page()
> > >>> memory_failure_hugetlb()
> > >>>   get_hwpoison_page(page)
> > >>>     __get_hwpoison_page(page)
> > >>>       get_page_unless_zero(page)
> > >>>                                   zero = put_page_testzero(page)
> > >>>                                   VM_BUG_ON_PAGE(!zero, page)
> > >>>                                   enqueue_huge_page(h, page)
> > >>>   put_page(page)
> > >>>
> > >>> The refcount can possibly be increased by memory-failure or soft_offline
> > >>> handlers, we can trigger VM_BUG_ON_PAGE and wrongly add the page to the
> > >>> hugetlb pool list.
> > >>
> > >> The hwpoison side of this looks really suspicious to me. It shouldn't
> > >> really touch the reference count of hugetlb pages without being very
> > >> careful (and having hugetlb_lock held).
> > > 
> > > I have the same feeling, there is a window where a hugepage is refcounted
> > > during converting from buddy free pages into free hugepage, so refcount
> > > alone is not enough to prevent the race.  hugetlb_lock is retaken after
> > > alloc_surplus_huge_page returns, so simply holding hugetlb_lock in
> > > get_hwpoison_page() seems not work.  Is there any status bit to show that a
> > > hugepage is just being initialized (not in free hugepage pool or in use)?
> > > 
> > 
> > It seems we can also race with the code that makes a compound page a
> > hugetlb page.  The memory failure code could be called after allocating
> > pages from buddy and before setting compound page DTOR.  So, the memory
> > handling code will process it as a compound page.
> 
> Yes, so get_hwpoison_page() has to call get_page_unless_zero()
> only when memory_failure() can surely handle the error.
> 
> > 
> > Just thinking that this may not be limited to the hugetlb specific memory
> > failure handling?
> 
> Currently hugetlb page is the only type of compound page supported by memory
> failure.  But I agree with you that other types of compound pages have the
> same race window, and judging only with get_page_unless_zero() is dangerous.
> So I think that __get_hwpoison_page() should have the following structure:
> 
>   if (PageCompound) {
>       if (PageHuge) {
>           if (PageHugeFreed || PageHugeActive) {
>               if (get_page_unless_zero)
>                   return 0;   // path for in-use hugetlb page
>               else
>                   return 1;   // path for free hugetlb page
>           } else {
>               return -EBUSY;  // any transient hugetlb page
>           }
>       } else {
>           ... // any other compound page (like thp, slab, ...)
>       }
>   } else {
>       ...   // any non-compound page
>   }

The above pseudo code was wrong, so let me update my thought.
I'm now trying to solve the reported issue by changing __get_hwpoison_page()
like below:

  static int __get_hwpoison_page(struct page *page)
  {
          struct page *head = compound_head(page);
  
          if (PageCompound(page)) {
                  if (PageSlab(page)) {
                          return get_page_unless_zero(page);
                  } else if (PageHuge(head)) {
                          if (HPageFreed(head) || HPageMigratable(head))
                                  return get_page_unless_zero(head);
                  } else if (PageTransHuge(head)) {
                          /*
                           * Non anonymous thp exists only in allocation/free time. We
                           * can't handle such a case correctly, so let's give it up.
                           * This should be better than triggering BUG_ON when kernel
                           * tries to touch the "partially handled" page.
                           */
                          if (!PageAnon(head)) {
                                  pr_err("Memory failure: %#lx: non anonymous thp\n",
                                         page_to_pfn(page));
                                  return 0;
                          }
                          if (get_page_unless_zero(head)) {
                                  if (head == compound_head(page))
                                          return 1;
                                  pr_info("Memory failure: %#lx cannot catch tail\n",
                                          page_to_pfn(page));
                                  put_page(head);
                          }
                  }
                  return 0;
          }
  
          return get_page_unless_zero(page);
  }

Some notes: 

  - in hugetlb path, new HPage* checks should avoid the reported race,
    but I still need more testing to confirm it,
  - PageSlab check is added because otherwise I found that "non anonymous thp"
    path is chosen, that's obviously wrong,
  - thp's branch has a known issue unrelated to the current issue, which
    will/should be improved later.

I'll send a patch next week.

Thanks,
Naoya Horiguchi

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